Even if the dead load isn't a problem, the space taken up by a one- to two-meter-deep soil level is going to be significant. (Oh, you were planning on occupying

Even if the dead load isn't a problem, the space taken up by a one- to two-meter-deep soil level is going to be significant. (Oh, you were planning on occupying the floor under the tree? Tough.) Also, even if draining the water isn't a problem, pumping all that water up there in the first place doesn't sound cheap.